一階層ドロップダウン（ホバー）

デモ

See the Pen nav 15 by logsuke (@design-list) on CodePen.

コード

<!-- Boxicons headタグ内へ -->
<link href='https://unpkg.com/boxicons@2.1.2/css/boxicons.min.css' rel='stylesheet'>

    <header class="header" id="header">
        <nav class="nav">
            <a href="#" class="logo">
                <svg viewBox="0 0 496 512" width="100" title="meh-rolling-eyes">
                    <path
                        d="M248 8C111 8 0 119 0 256s111 248 248 248 248-111 248-248S385 8 248 8zM88 224c0-24.3 13.7-45.2 33.6-56-.7 2.6-1.6 5.2-1.6 8 0 17.7 14.3 32 32 32s32-14.3 32-32c0-2.8-.9-5.4-1.6-8 19.9 10.8 33.6 31.7 33.6 56 0 35.3-28.7 64-64 64s-64-28.7-64-64zm224 176H184c-21.2 0-21.2-32 0-32h128c21.2 0 21.2 32 0 32zm32-112c-35.3 0-64-28.7-64-64 0-24.3 13.7-45.2 33.6-56-.7 2.6-1.6 5.2-1.6 8 0 17.7 14.3 32 32 32s32-14.3 32-32c0-2.8-.9-5.4-1.6-8 19.9 10.8 33.6 31.7 33.6 56 0 35.3-28.7 64-64 64z" />
                </svg>
                logo
            </a>
            <div class="nav-menu" id="navMenu">
                <ul class="nav-links">
                    <li class="nav-item">
                        <button>Section1</button>
                        <ul class="drop-down">
                            <li class="item"><a href=""><img src="https://placehold.jp/969ce8/ffffff/200x150.png" alt=""></a></li>
                            <li class="item"><a href=""><img src="https://placehold.jp/969ce8/ffffff/200x150.png" alt=""></a></li>
                            <li class="item"><a href=""><img src="https://placehold.jp/969ce8/ffffff/200x150.png" alt=""></a></li>
                            <li class="item"><a href=""><img src="https://placehold.jp/969ce8/ffffff/200x150.png" alt=""></a></li>
                        </ul>
                    </li>
                    <li class="nav-item">
                        <button>Section2</button>
                        <ul class="drop-down">
                            <li class="item"><a href=""><img src="https://placehold.jp/969ce8/ffffff/200x150.png" alt=""></a></li>
                            <li class="item"><a href=""><img src="https://placehold.jp/969ce8/ffffff/200x150.png" alt=""></a></li>
                        </ul>
                    </li>
                    <li class="nav-item">
                        <button>Section3</button>
                        <ul class="drop-down">
                            <li class="item"><a href=""><img src="https://placehold.jp/969ce8/ffffff/200x150.png" alt=""></a></li>
                            <li class="item"><a href=""><img src="https://placehold.jp/969ce8/ffffff/200x150.png" alt=""></a></li>
                            <li class="item"><a href=""><img src="https://placehold.jp/969ce8/ffffff/200x150.png" alt=""></a></li>
                            <li class="item"><a href=""><img src="https://placehold.jp/969ce8/ffffff/200x150.png" alt=""></a></li>
                            <li class="item"><a href=""><img src="https://placehold.jp/969ce8/ffffff/200x150.png" alt=""></a></li>

                        </ul>
                    </li>
                    <li class="nav-item">
                        <button>Section4</button>
                        <ul class="drop-down">
                            <li class="item"><a href=""><img src="https://placehold.jp/969ce8/ffffff/200x150.png" alt=""></a></li>

                        </ul>
                    </li>
                </ul>
            </div>

            <div class="nav-toggle" id="navToggle">
                <i id="navIcon" class='bx bx-menu'></i>
            </div>
        </nav>
    </header>

/*=== Base ===*/

* {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
  font-family: sans-serif;
}

:root {
  --nav-height: 8rem;
  --max-width: 1024px;
  --container-padding: 0 4rem;
  --gap: 2rem;
  --bg-color: rgb(240, 240, 240);
}

/*=== Font ratio ===*/

html {
  scroll-behavior: smooth;
  font-size: 62.5%;
}

body {
  font-size: 1.6rem;
}

ul {
  list-style: none;
}

a {
  text-decoration: none;
}

button {
  border: none;
  font-size: 1.6rem;
}

/* header */
header {
  width: 100%;
  position: fixed;
  top: 0;
  left: 0;
  z-index: 100;
  background-color: var(--bg-color);
}

.nav svg {
  width: 25px;
  padding: 2px;
  margin-right: 5px;
}

/* nav */
.nav {
  width: 100%;
  max-width: var(--max-width);
  height: var(--nav-height);
  display: flex;
  justify-content: space-between;
  align-items: center;
  margin: 0 auto;
  padding: 0 3rem;
}

.logo,
.nav-toggle {
  color: var(--text-color);
  cursor: pointer;
}

.logo {
  font-weight: bold;
  font-size: 2rem;
  z-index: 100;
  display: flex;
  justify-content: center;
  align-items: center;
}

.nav-toggle {
  font-size: 3rem;
  display: flex;
  align-items: center;
  z-index: 200;
}

.nav-toggle i {
  display: none;
}

.nav-links {
  display: flex;
  align-items: center;
  justify-content: center;
  gap: 2rem;
}

.nav-links>a:hover {
  color: rgb(134, 170, 207);
}

.nav-item>a, .nav-item>button {
  color: var(--text-color);
  display: flex;
  justify-content: center;
  align-items: center;
  height: var(--nav-height);
  border: none;
  font-size: 1.6rem;
  cursor: pointer;
}

.nav-item:hover {
  color: rgb(134, 170, 207);
}

.nav-item:hover .drop-down {
  opacity: 1;
  pointer-events: auto;
}

.drop-down {
  width: 100%;
  position: absolute;
  top: var(--nav-height);
  left: 0;
  background: var(--bg-color);
  padding: 3rem;
  display: flex;
  justify-content: space-around;
  flex-wrap: wrap;
  opacity: 0;
  pointer-events: none;
  transition: opacity .2s;
}

.item {
  padding: 2rem 0;
  position: relative;
}

.item a {
  display: flex;
  align-items: center;
  justify-content: center;
  background-color: var(--first-color);
  overflow: hidden;
}

.item img {
  width: 100%;
  object-fit: cover;
  transition: transform .4s;
}

.item:hover img {
  opacity: .6;
  transform: scale(1.2);
}

const navMenu = document.getElementById('navMenu'),
    navToggle = document.getElementById('navToggle'),
    navIcon = document.getElementById('navIcon'),
    navLink = document.querySelectorAll('.nav-link')

navToggle?.addEventListener('click', function () {
    if (navIcon.classList.contains('bx-menu')) {
        navIcon.classList.replace('bx-menu', 'bx-x');
        navMenu.classList.toggle('toggle');
    } else {
        navIcon.classList.replace('bx-x', 'bx-menu');
        navMenu.classList.toggle('toggle');
    }
})

navLink.forEach(function (li) {
    li.addEventListener('click', function () {
        navIcon.classList.replace('bx-x', 'bx-menu');
        navMenu.classList.remove('toggle');
    })
})